1. Field of the Invention
The present invention relates to a reversible electrode material, a method for producing a reversible electrode using the electrode material, and a lithium battery using the reversible electrode.
2. Description of the Prior Art
Since conductive polyacetylene was discovered by Shirakawa et al. in 1971, the use of this conductive polymer as an electrode material has been extensively studied. This is because electrochemical devices such as lightweight batteries with a high energy, electrochromic elements having a large area, and biochemical sensors using minute electrodes can be expected by using the conductive polymer as the electrode material. However, since polyacetylene has some disadvantages for practical use because of its chemical instability, other .pi. electron conjugated conductive polymers, which are relatively stable, such as polyaniline, polypyrrole, polyacene, and polythiophene have been studied. Lithium secondary batteries using these polymers for their cathodes have been developed. Since these polymer electrodes take in anions existing in the electrolyte as well as cations while they are charged or discharged, the electrolyte in the battery not only functions as a medium for ion conduction but also participates in the battery reaction. Therefore, it is required that the electrolyte is supplied to the battery in an amount corresponding to the battery capacity. The energy density of the battery is limited in the range of about 20 to 50 Wh/kg, which is nearly equal to or lower than that of conventional secondary batteries such as nickel-cadmium or lead-acid batteries.
As an electrode material which can provide a higher energy density, a disulfide compound has been disclosed in the U.S. Pat. No. 4,833,048. This compound is most simply represented by R-S-S-R (where R is an aliphatic or an aromatic organic group and S is a sulfur atom). An S-S bond is cleaved by the electrolytic reduction in an electrolytic cell containing cation of M.sup.+ to form a salt represented by R-S.sup.-. M.sup.+. This salt returns to the R-S-S-R by the electrolytic oxidation. In the U.S. Pat. No. 4,833,048, a metal-sulfur type rechargeable battery obtained by combining a disulfide compound with metal M which supplies and captures the cations (M.sup.+) has been proposed. According to this rechargeable battery, an energy density of 150 Wh/kg or more, which is much more than that of conventional secondary batteries, can be expected.
As described above, the disulfide compound is proposed as an electrode material which can provide a higher energy density. However, as the inventors of U.S. Pat. No. 4,833,048 reported in J. Electrochem. Soc, Vol. 136, No. 9, pp. 2570 to 2575 (1989), the difference between the oxidation potential and the reduction potential is very large. For example, when [(C.sub.2 H.sub.5).sub.2 NCSS-].sub.2 is electrolyzed, the oxidation potential differs from the reduction potential by 1 V or more. According to the electrode reaction theory, the electron transfer of the disulfide compound proceeds extremely slowly. Because of this, it is rather difficult to obtain a rechargeable battery providing a higher current output of 1 mA/cm.sup.2 or more at room temperature. The operation of a battery comprising an electrode of disulfide compound is limited to a high temperature in the range of 100.degree. to 200.degree. C., where the electron transfer can proceed faster.